My advice is to never trust polls. Studying marketing, I have learned it is quite easy to warp the data that they received. Like, oh, maybe leaving out people under 25. Or not putting in the votes of the blacks.
And in all honestly, unless they asked each and every American, they have no clue about percentages.
As for Catholics and Protestants who 'claim' they are Christian. I know a heck of a lot of them that aren't practicing Christians. Now while some do believe in God, but don't go to Church, most of them don't believe. And many Churchgoers, such as my grandmother, only go in case there really is a God, so that they won't go to heck. :|